Method of purifying caustic alkali solution



Patented July 9, 1946 METHOD OF PURIFYIN'G CAUSTI ALKALI' SOLUTION Claude A. Cummins, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich,

a corporation of Michigan No Drawing. Application June 2, 1943, Serial No. 489,387

6 Claims. 1

This invention concerns an improved method of purifying caustic solutions, particularly of removing chlorates therefrom. t

It is known that sodium and potassium hydroxide solutions, as obtained by the electrolysis of corresponding alkali metal chloride-containing brines, usually contain a small amount of the alkali. metal chlorate and that the presence of the chlorate frequently is objectionable. Certain methods for removing the chlorate, e. g.by dehydrating the alkali and heating it to a temperature above that at which the chlorate is decomposed or by subjecting the alkali solution to electrolysis under certain special conditions, are known, but these previously known methods are have reacted to reduce the chlorate to the chloinconvenient or diflicult to carry out and add appreciably to the cost of the caustic product.

It has long been known that a chlorate may be reduced, while in a neutralor acidic solution, by agitating the same with finely divided iron, but the literature on the subject indicates that such reduction would occur sluggishly and in completely, or not at all, in an alkaline solution.

study on the corrosive action of aqueous alkali.

solutions toward iron and certain of its alloys, show that the corrosion of iron occurs in two ways, i. e. with and without evolution of hydrogen; that chlorates, if present, may accelerate the rate of corrosion; but that iron in the presence of alkali and a chlorate often behaves asif it were passive and in these instances the corrosion was. least. The experimental data in this article by Rudensky shows that the corrosion of iron which occurred was in all instances such as to reduce very little, if any, of the chlorate in the alkali solution. For instance, the weight of iron lost by corrosion per part of chlorate in the alkali solution was greatest inthe experiment in Table 6 of the article on the actionof chlorate-containing caustic soda on an iron strip which was immersedin the solution for 6 hours while heating at 90. C. In this.v experiment, if all of theiron lost by corrosion. isassumed to.

ride (and the article indicates that at least part of the corrosion may be due to other causes) the chlorate thus destroyed would amount to only 9 per cent of that originally present in the solution, leaving at least 91 per cent of the chlorate in the solution.

This showing by Rudensky that an extended surface, e. g. a strip, of iron will not cause appreciable reduction of a chlorate in an alkali solution and, in fact, will often be rendered passive and corrosion-resistant during contact with such alkaline chlorate-containing solution accords with the facts observed over a period of many years in the manufacture of caustic by the electrolytic process. In such usual manufacturing process, the crude chlorate-containing caustic solution, as obtained from the electrolytic cells, is evaporated at elevated temperatures, stored, shipped, and otherwise handled in iron or steel equipment, but such prolonged contact of the solution with iron surfaces results in very little, or no, reduction in the chlorate content of the solution. Furthermore, it has been-observed that when iron apparatus is first placed in service in such manufacturing process, corrosion sometimes occurs to a noticeable extent without, however, appreciable reduction in the chlorate content of the solution, but that after the apparatus has been in service for a short time the rate of corrosion decreases, indicating that the iron has been rendered resistant to corrosion by the solution.

I have now found that although iron having extended surfaces is not effective in reducing the chlorate in an aqueous. caustic alkali solution and, in fact, is often rendered passive and corrosion-resistant during contact with such solution, finely divided iron, e. g. iron powder or filings, is highly effective in reducing chlorates to chlorides in the presence of alkali. I have further found that the reduction of the chlorate with finely divided iron occurs more rapidly and is more readily carried to completion when the chlorate-containing caustic solution is of quite high concentration, e. g. when it contains from 30 to per cent by weight of sodium or potassium hydroxide, than when it is of lower concentration. This is contrary to the result expected in view of the repeated showing in the prior art that the reduction of halates usually occurs more readily in acidic solutions than under alkaline conditions.

However, the treatment of an aqueous caustic alkalisolution. with finely divided iron usually results in the introduction of iron-containing impurities into the solution, and in discoloration of the latter. For instance, a colorless chloratecontaining caustic alkali solution which initially contains less than 0.01 per cent of iron, based on the dry weight of the alkali metal hydroxide, after being treated with finely divided iron to destroy the chlorate is of greenish color and usually contains from 0.03 to 0.12 per cent of dissolved iron, based on the dry weight of the alkali metal hydroxide. The iron is presumed to be present as an alkali metal ferrate or ferrite, but the identity of the iron-containing impurity has not definitely been established.

I have found that the dissolved or disperse iron-containing impurity may readily be removed by treating the caustic alkali solution at a low or moderate temperature with a minor amount of chlorine or a soluble hypochlorite. By this treatment the iron-containing impurity is converted into an insoluble white, substance which may be removed by filtration. The precipitation of the iron-containing impurity is accompanied by a disappearance of the greenish color and this color change serves as a convenient indicator as to the amount of chlorinev or hypochlorite required to complete the precipitation.

The invention then comprises two new steps; viz., that of'destroying a chlorate occurring in' an aqueous caustic alkali solution by treating-the solution with finely divided iron and that of subsequently removing dissolved or dispersed iron compounds from the solution by treating the latter with a minor amount of chlorine or a hypochlorite.

In order to be satisfactory for use in the first of these steps, i. e. that of destroying a chloram, the iron must be of a physical form such that the ratio of the square millimeters total surface of the metal to its cubic millimeters absolute volume is or higher, and preferably above 30. Iron powder or filings of 100 mesh particle size or finer is'preferably used. The iron may be in the form of a powder of wrought iron, or cast iron, or steel. The iron or steel should be of a form and composition susceptible to corrosion in the presence of moisture and oxidizing agents.

In practicing the invention, a caustic soda or a caustic potash solution of between 30 and 60, and preferably between 415 and 55 per cent by weight concentration and containing a soluble chlorate is treated with the iron powder, preferably in amount exceeding that theoretically required to reduce the chlorate to the chloride. The iron powder'isefiective in causing reduction of the chlorate regardless of the amount of iron employed and, when substantially complete. removal of the chlorate is not desired it may, of course, be used in a smaller proportion, v I

Reaction between the iron powder and the alkali metal chlorate in the caustic solution to destroy the chlorate takes place smoothly, but quite slowly, on permitting the mixture to stand at room temperature. It occurs fairly rapidlyat temperatures from 70 C. to the boiling temperature of the caustic solution and is advantageously carried out at temperatures within this range, preferably at temperatures above 80 C. By operating at the preferred temperatures just given, 90 per cent or more of the chlorate impurities ordinarily present in caustic solutions may usually be destroyed in less than 3 hours. 7

After completing the reaction, the mixture is filtered to remove unreacted iron powder. The filtrate is a greenish colored caustic alkali solution which, although substantially free of chlorate,

usually contains from 0.03 to 0.12 per cent of iron (presumably as a ferrate or ferrite) based on the dry weight of the alkali metal hydroxide.

The dissolved or dispersed iron compounds are precipitated by treating such iron-containing causticalkali solutionwith a minor amount of chlorine or a hypochlorite, e. g. a solution of chlorine in cold water or in a cold aqueous alkali metal hydroxide solution, preferably while agitating the mixture. The treatment is carried out at a temperature below C., and preferably between 15 and 25 (2., since the precipitation of the iron compounds occurs most favorably at these moderate or low temperatures. During the treatment, a white precipitate is formed and the greenish color of the caustic alkali solution disappears, 1. e. the mixture becomes white or colorless. After completing the treatment, the precipitate is removed, e. g.. by filtration, preferably while at a temperature below 30 C. The caustic alkali solution obtained as the filtrate is substantially colorless. It retains only a minute amount of iron-containing compounds, e. g. an amount corresponding toless' than 0.01 per cent, and usually less than 0.0005 per cent, of the dry weight of the alkali metal'hydroxide.

' When the caustic alkali solution subjected to the treatment is subsequently to be concentrated I by evaporation in nickel apparatus, it is important that the employment of excess chlorine or hypochlorite over the amount required to precipitate the iron compounds be avoided, since the presence of a hypochlorite, or other oxidizing agent, during such evaporation may result in corrosion of the apparatus. The addition of chlorine or a hypochlorite to the iron-containing caustic alkali solution may be controlled by periodically withdrawing a portion of the solution and analyzing it for dissolved iron. This is a somewhat inconvenient method for controlling the addition to avoid'the employment of an excess of chlorine or a hypochlorite, but when the treatment is carried out ina closed container so that the mixture can notbeobserve'd ,during 'the addition, it may be initial green color of the caustic alkali solution form whichis as free, and'u'sually is freenof disdisappears when precipitation of the iron-containing compounds is substantially complete, i. e. when the ironcompounds have been precipitated as'completely as possible by the present method. The disappearance of thegreenish color occurs quite sharply at temperatures below 30 C. and serves as a convenient indicator in determining the amount of chlorine or hypochlorite to be used. By discontinuing the addition when the green color disappears, the employment of an excess of chlorine or a hypochlorite may be avoidedfi In some-Q instances 1 the" treatment is discontinued when theeolor of the caustic alkali solution has become'faint, but is still discernible. In case an excess of chlorine orahypochlorite. is added during the step of. precipitating the iron-containing impurity, .it may be removed by, adding to the mixture a further amount of the iron-containing caustic alkali solution.

By operating as just described, fairly concentrated' aqueous caustic solutions which are liquid at room temperature and which contain analkali metal chlorate may readily be freed. of the latter and be recovered in a substantially colorless soivadr d spersed n cbni ounds tha i i before tnetreatment- I:

Thelfollowing examples illustrate certain ways in which the principle'of the invention has been applied, but are not to be construed as limiting the invention. 4

The purpose of 'thisexample isto show thatthe reduction with iron'powder of a chlorate in an aqueous caustic, 'soda/solutionj'occurs more rapidly and completely and requires asmall'er excess of, the'iron for, rapid nd complete reaction when the; caustic solution 1s fai r1y eoncentrat than when it-is" dilute. Thref samplesjof. caustic soda solution whichfwere-collected abdifierent, sta es in the ,electrolytic'process'. for the manufacture of caustic soda and were of difiere'nt concentrations were employed. A thoughthe solutions were .of different concentrations, they each contained so; diuinl chlorate in amount corresponding to approximately 0.156. per cent of the weight of the sodium hydroxidej'present thereinQ-Each solu.- tion wastreated with 100 mesh iron powder in the amount indicated; in the following table and the mixture was heated at the temperature and for the time also given. The treated solutions were of greenish color. After completion of the heating operation each solution was analyzed to determine its chlorate content, whereby the proportion of the chlorate initially present which had been destroyed by treatment with the iron powder was determined. The table gives the concentration of each caustic soda solution as per cent by weight of sodium hydroxide therein and the proportion of iron powder added in terms of pounds of iroii per: tonfof caustic'solution. Each mixture'was stirred continuously, one at temperatures-between 90 3 and 100 C. and the. other at approximately 145 'C., and aliquot portions at said temperature.

the pounds of iron powderper ton dry weight 7 of the sodium hydroxide. .It also gives the temperature and time of heating in each experiment and the per cent by weight of the chlorate initially present in each caustic solution which was removed by the treatment with iron powder.

A 50 per cent by weight concentrated aqueous caustic soda solution which contained approximately 0.08 per cent by weight of sodium chlorate was treated with an approximately 100 mesh iron powder in amount corresponding to 8 pounds of iron per ton of the solution. The mixture was stirred at room temperature to maintain the iron distributed throughout the same and aliquot portions of the solution were withdrawn from time to time and analyzed to determine the chlorate content thereof. It was found that 6.6 per cent of the chlorate originally present was destroyed in 2 hours and that 72 per cent of the original chlorate was destroyed in 24 hours. The treated solutions were of greenish color.

EXAMPLE 3 Each of two samples of a 50 per cent concentrated caustic soda solution containing approximately 0.08 per cent by weight of sodium chlorate was treated with powdered iron of 100 meshparticle size in amount corresponding to 8 of" each mixture were withdrawnirom time to time and analyzed to determine the chlorate-content thereof. 'After completing the treatment eachsolutio'n was'of greenish color. -'It was found that 93" per cent ofthe chlorate in one'of -the samples'was destroyed by {3 hours of heating at l00 C. in the presence of the iron. A It was also foundthat 94 per cent of the chlorate originally present in theother samplewasdestroy'ed by 5- minutes of he'atingofth'e sample with the iron at 145 C., and that all of the'chlorate'had been "removed after only 15- minutesof heating A 60-ton batch of a- 50 per centconcentrated aqueouscaustic sodasolution which contained 0.25s per cent by weight of sodium chlorate was treated with 480 pounds of iron powder ofsmaller than mesh particle sizeand the mixture was heat'e'd 'at temperatures between"90 and"l00 C. for 2.5 hours. An additional 25 pounds of iron powder was then added and the heating was continued for 0.5 hour. The mixture was cooled and iron and insoluble iron compounds suspended therein were removed. The treated liquor, which was of greenish color, contained only 0.0047 per cent by weight of sodium chlorate.

EXAMPLE 5 A greenish colored aqueous caustic soda solution of 50 per cent concentration, and containing approximately 500 parts per million of dissolved iron (i. e. iron in amount corresponding to 0.10 per cent of the dry weight of the alkali), was treated with chlorine until colorless. This treatment, which was carried out with agitation, re sulted in the formation of a white precipitate of an iron-containing compound. After adding the chlorine, the mixture was agitated for three hours and then filtered. The filtrate was a colorless caustic soda solution containing only 15 .parts per million of iron.

This application is a continuation-in-part of my copending application, Serial No. 462,417, filed October 17, 1942, which is, in turn, a continuation-in-part of an earlier application, Serial No.- 375,257, filed January 21, 1941,

Other modes of applying the principle of the invention may be employed instead of those explained, changebeing made as regards the meth.. od herein disclosed, provided the step or steps stated by any oi the following claims or the equivalent of such stated step or steps be employed.

I therefore particularly point out and distinctly claim as my invention:

1. In a method of treating a chlorate-containing aqueous caustic alkali solution to remove the chlorate, the steps of contacting the solution with finely divided iron and destroyin the chlorate by reaction'with the finely divided iron, introducing into th resultant caustic alkali solution, which is enriched in iron compounds and is of greenish color, while the solution is at a temperature below 30 C., an agent selected from the class consisting of chlorine and soluble hypochlorites, said agent being employed in amount suiiicient to render the solution substantially olorless, and removing the iron-containing substance which is thereby precipitated.

' 2. Inamethodoitz eatlnz achloratewontain: ing aqueous caustic alkali solution or vn'ornoxtimtvo 60. Per cent concentration to remove. the chlorate, the steps ofeontacting, the solution wlth flnely' divided iron and destro g. the chlorate by heatin the ..solution together with the finely volivided f iron at a temperature between 70 C. and the boilins tempex ature thereafter filtering the mlxture, introducinginto the resultant solution, Whlohls of greenish color and- 1s enrlohedin iron com;

e ow 0- an agent e ed om he- 18 5. ens sfifn of chlozine dso u emoch teo. ai s n eina m ov di x tel t amount requireq to render the solution substan el y. e s.- d em vi e-thei on-wnt nc substan'e which is thereby prcipitated,-

3. The method as described in'claim 2 wherein the caustic alkali is sodium hydroxide and the chlorateis sodium chlorate.

4; Ina method of treating an aqueous caustic alkalisolution to remove impurities therefrom, the stefi of precipitating an iron compoundfrom the solution by introducing. into the latter; while at atempei-ature below 30 0., an agent selected 7 poundsLwhi-le the .solution is at a temperature by treatment w th fine y dividedi bais. 021mm:

h lor nd, o tains m re eulfiom rtspe l ion. of ma n. he 913 .9 .dl ersedltbn om: m m by tmduqma nt he olut n w e at a tengpemtuge between 15}? and 25 c -chlonne in amount sumolent-to lender the lnlictute all! stantlallylcolox lesg anel thexeafte removingvthe n com n h h a s. e ip t d ;by th s 

